The integration of MOSFET structures and bipolar transistors on a single substrate has become very desirable. As is well known in the art, digital and linear functions are often performed by integrated circuits using either bipolar or metaloxide semiconductor (MOS) technology. Bipolar integrated circuits, of course provide higher speed operation and greater drive currents than the MOS circuits, at the cost of higher power dissipation, especially when compared against complementary MOS (CMOS) circuits. Advances in manufacturing technology have allowed the use of both bipolar and CMOS transistors in the same integrated circuit. These are commonly referred to as BiCMOS devices.
In a BiCMOS device, bipolar transistors are typically formed by using a moat region for the base of the bipolar transistor, doped polysilicon for the emitter, and a well region for the collector. Resistors typically comprise either a diffused region or polysilicon which is doped n-type. CMOS circuits comprise both NMOS and PMOS transistors. In order to scale the PMOS transistor and provide low threshold voltages, it is desirable to use a surface channel PMOS transistor. Thus, NMOS and PMOS transistors use doped polysilicon for the gates and diffused regions for the source/drain regions. This complicates the process by requiring both n+ and p+ polysilicon gates that are typically doped by the source/drain implants.
The source/drain implants, however, may not provide adequate doping through the polysilicon gates to the gate oxide. Adequate doping of the NMOS gates can be achieved by implanting the NMOS gates during the bipolar emitter implant. Improved doping of the PMOS gates may be achieved by scaling the polysilicon thickness. However, scaling the polysilicon thickness is undesirable in a BiCMOS process that uses the same polysilicon layer for the emitter and the NMOS and PMOS gates. Accordingly, there is still a need for adequate doping through the polysilicon to the gate oxide in a PMOS transistor that does not require the polysilicon thickness to be scaled.